High-speed data services such as Digital Subscriber Line (“DSL”) service, and its variations such as ADSL, HDSL, SDSL, VDSL etc. (referred to herein collectively as “xDSL” or simply “DSL”), are becoming popular methods of communicating large volumes of data over standard twisted pair copper telephone wiring, for example for Internet data communications and World Wide Web browsing applications. DSL communications techniques have become common in many urban areas as an option for high-speed data service.
In a DSL communications service, a subscriber who pays for the DSL service is generally connected to a DSL network at a local telephone company central office. The DSL network exchanges data with the subscriber, in digital format, over the same twisted pair copper telephone wires which provide voice telephony service (commonly known as “plain old telephone service” or “POTS”), but at a frequency higher than the band reserved for analog service and with sufficient signal separation that there is optimally no interference between the DSL signals and analog telephony signals, so that both can be carried simultaneously. Typically the DSL service is constantly connected to the local telephone DSL distributor, while the POTS service is connected only when a telephone device is in use.
In an xDSL system each subscriber is served by a DSL port mounted on a compact (or other form factor) PCI line card. The line card is itself mounted in a rack which is fed by a large bandwidth communications carrier, typically an optical fiber, currently carrying data at rates of up to 200 Mbps. Each PCI card provides DSL chips comprising a modulator/demodulator, for example using DMT or QAM modulation (referred to herein as a “network modem”). Each DSL line card is currently provided with four network modems. Each network modem is dedicated to one communications port, to thereby serve four separate subscribers in point-to-point fashion, and is disposed in a rack containing 14 line cards. Thus, each rack is typically able to serve 56 subscribers.
Because each DSL port serves only a single subscriber this low DSL port density is very uneconomical, costing the telephone service in the order of thousands of dollars per subscriber, and as such making DSL service relatively unappealing to DSL service providers. Also, DSL racks occupy a significant amount of space, and the space available in a telephone company central office is limited. The lack of physical space available for adding new DSL racks limits the ability of the telephone company DSL service provider to expand its own customer base, and in some cases is used as a justification for excluding competition by other DSL service providers.
Moreover, the 200 Mbps data stream fed into the rack must be divided amongst all DSL subscribers, thereby limiting the DSL data output to less than 4 Mbps per port. Accordingly, despite the value to the subscriber of the DSL service, telephone companies have been slow to expand DSL services because the per subscriber cost makes it difficult to earn a return on their investment.
While it is possible to increase the DSL port density of each PCI card to some extent, and thereby decrease the per port cost of DSL installations to the telephone company, there remains the limitation that the rack input data stream must be divided amongst all subscriber ports, so the rack input data rate will ultimately limit the practical DSL port density of the rack. There is still much bandwidth available for downloads to subscribers, since DSL ports are currently operating at well below capacity; however, this bandwidth must be shared with upstream communications from the subscriber to the telephone company.
By far the most common DSL service, especially for consumer and small business service, is asymmetric DSL (“ADSL”). Unlike symmetric DSL (SDSL) and high bit-rate DSL (HDSL), which are symmetric diplex systems, ADSL is asymmetric because most of the diplex bandwidth is devoted to downstream communications (the term “downstream” as used herein meaning data communication from the DSL distributor to the subscriber), while only a small portion of bandwidth is made available for upstream data communications containing subscriber requests and uploads (the term “upstream” as used herein meaning data communication from the subscriber to the DSL distributor). This is a convenient method of managing DSL bandwidth, since upstream and downstream communications must share the available bandwidth and most Internet content contains graphics- and multi-media intensive-data which requires substantial downstream bandwidth, whereas subscriber requests and uploads have typically utilized much less data and thus required significantly lower upstream bandwidth. A typical ADSL communications system thus provides between 3 and 4 Mbps of downstream data and only 640 kbps of upstream data.
However, subscriber demands are changing. As digital photography, videoconferencing, e-commerce and other consumer/small business applications become more popular, upstream data demands are becoming significantly more data intensive, to the point that upstream bandwidth has become an important limitation on the capacity of DSL services. At the central office end, DSL service providers can connect multiple DSL subscribers to a high-speed backbone network using a Digital Subscriber Line Access Multiplexer (DSLAM), which multiplexes a plurality of data streams at high data rates while at the receiving end a DSLAM demultiplexes the signals and sends them to the individual destination subscriber modems. However, this does not address the data stream limitations of the so-called “last mile,” between the DSL equipment and DSL subscribers, which remains a point-to-point communications system in which one line can serve only one subscriber modem. Thus, for a subscriber to implement multiple stations utilizing the DSL service requires expensive networking equipment.
There are two obstacles associated with increasing the number of subscriber modems per subscriber line. First, because the DSL line also carries analog voice telephony signals, it is not possible to directly couple the telephone lines of different subscribers. This would effectively turn the coupled subscribers' lines into a “party line”. This problem can be overcome by bridging the separate subscriber lines through a spectrally differential bridging structure, for example the bridged data distribution network described in copending U.S. patent application Ser. No. 09/702,759 to Jeffery filed Nov. 1, 2000, which is incorporated herein by reference. This system couples the subscribers' lines only at frequencies above the analog telephony band. The coupling is invisible to the subscribers because low frequency POTS interconnection between subscriber lines remains blocked by filters, while communications at the coupled frequencies are packet-switched so each subscriber receives only packets intended for that subscriber.
The second obstacle is based on data management. Because upstream and downstream data transfers must share the same bandwidth, whatever bandwidth is used for uploads to the communications distribution rack is not available for downloads to subscribers. This precludes the provision of multiple subscriber modems on a subscriber line, even within the same subscriber premises, since there is no means for managing upload data from multiple subscriber modems on a single subscriber line. Thus, so-called “wide area network” communications systems such as DSL are currently constrained to a point-to-point communications architecture.
There is accordingly a need for a system and method for enabling point-to-multipoint communications in a network communications system, such as DSL, which allows a greater number of modems to share a single line connected to the DSL rack and thus to be connected to a single data port. This would not only substantially decrease the per-subscriber cost to the service provider, but would also effectively eliminate physical space constraints as a limitation on subscriber capacity.